1. Field of the Invention
The present invention relates to an apparatus and method for operating a wavelength converter. More particularly, the invention relates to an apparatus and method to obtain a maximum extinction ratio by feeding back a portion of an optical signal output from a Mach-Zehnder interferometer wavelength converter using cross phase modulation (XPM) of a semiconductor optical amplifier to maintain an optimum phase difference between both arms.
2. Description of the Related Art
In order to increase a capacity and transmission speed of a communication network, efficiently operate the communication network, and improve the reliability of the network, communication environments are varied in a manner that individual networks are integrated into an integrated network. This requires large-capacity information superhighways.
A wavelength division multiplexing (WDM) technique using all-optical wavelength conversion, one type of communication infra-technology to meet the aforementioned requirement, may improve a transmission capacity of a communication network. Also, this technique enables the construction of a variable network because it uses wavelengths as routing information. In a WDM network, data is transmitted to nodes through wavelength paths that are determined by wavelengths used. However, an information connection between networks that use different wavelengths, or the number of wavelengths that may be used for each channel in the network, is limited. Accordingly, wavelength collision occurs when different channels use the same wavelength, and the designation of a path for each wavelength and effective routing become difficult, which may cause problems in the network system.
To effectively operate the WDM all-optical network, optical cross connection is required for each node. The optical cross connection is accomplished through an optical cross connection (OXC) technique. The OXC is performed by a wavelength converter that converts wavelengths, a space switch that routes a wavelength path, an optical add drop multiplexer (OADM) that drops or adds the path of an input/output optical signal at a node, and a MUX/DEMUX that combines or distributes signals with different wavelengths. The wavelength converter is the core element that implements the OXC in the WDM network. Wavelength conversion is a technique that transmits data, which is input with a certain wavelength, at a newly allocated wavelength. The wavelength conversion technique includes cross-gain modulation (XGM), cross-phase modulation (XPM), and 4 wave-mixing (FWM), that use non-linearity of a semiconductor optical amplifier.
FIG. 1 is a block diagram of a conventional XPM wavelength converter that employs a Mach-Zehnder interferometer type semiconductor optical amplifier. The XPM wavelength converter includes a first semiconductor optical amplifier 100, a second semiconductor optical amplifier 101, a π phase shifter 102, and an optical band-pass filter 103.
In a semiconductor optical amplifier, when a small signal is applied to a semiconductor active layer while the active layer is in a density-inverted state due to current injection, a signal that is amplified by stimulated emission caused by free electrons of a conduction band combining with holes of a valence band is output as photons.
The first semiconductor optical amplifier 100 amplifies a pump signal Ppump and a probe signal Pprobe according to a current i1 and phase-shifts the signals by XPM. Here, the pump signal Ppump is a modulated optical pulse signal having a wavelength λ1, as shown in FIG. 2A, and the probe signal Pprobe is a continuous wave optical signal having a wavelength λ2, as shown in FIG. 2B. The part of the conventional XPM wavelength converter of FIG. 1 that amplifies the pump signal and probe signal by the first semiconductor optical amplifier 100 is designated as a first arm.
The second semiconductor optical amplifier 101 amplifies the probe signal according to a current i2. The π phase shifter 102 phase-shifts the signal amplified by the second semiconductor optical amplifier 101. The part of the conventional XPM wavelength converter of FIG. 1 that amplifies the probe signal through the second semiconductor optical amplifier 101 and phase-shifts the amplified signal using the π phase shifter 102 is designated as a second arm.
Here, the π phase shifter 102 creates a phase difference of π radians between the first and second arms to improve an extinction ratio. The optical band-pass filter 103 cuts off the pump signal and outputs only the probe signal.
FIG. 3 illustrates waveforms of probe output signals of the optical band-pass filter 103 for two different phase shifts of the π phase shifter 102. In FIG. 3, (a) is a probe output signal when the phase difference between the first and second arms is zero and (b) is a probe output signal when the phase different is π.
In the Mach-Zehnder interferometer (MZI) structure, the phase of the second arm is retarded by π from the phase of the first arm due to the π phase shifter 102. The first semiconductor optical amplifier 100 has no phase delay at a low pump power, but has a phase delay of π due to XPM at a high pump power. Accordingly, the phase difference between the first and second arms is π at a low pump power and zero at a high pump power in the MZI structure. A probe power is subjected to interference due to the phase difference so that the probe signal and pump signal are modulated in the same manner. That is, constructive interference occurs when the phase difference between the first and second arms is 2nπ, and destructive interference occurs when (2n+1)π, where n=0, 1, 2, 3, . . . . According to the interference, the signal having a wavelength of λ1 is converted into a signal having a wavelength of λ2, to generate an output signal P°probe shown in FIG. 2C.
The conventional XPM wavelength converter employing a Mach-Zehnder interferometer semiconductor optical amplifier outputs a non-inverted signal with a high extinction ratio. However, the XPM wavelength converter may maintain the high extinction ratio only when the phase difference between the first and second arms is (2n+1)π at a low pump power and 2nπ at a high pump power. For this, the π phase shifter 102 is manually adjusted to set the phase difference accurately. Furthermore, the phase difference must be stabilized because it is very sensitive to the external environment.